Researchers from the University of São Paulo (USP) and the Institute of Environmental Research (IPA) have confirmed that a mysterious circular structure in São Paulo's Parelheiros district was created by the impact of an asteroid, ending decades of debate over its origin.
The structure, known as the Colônia Crater, lies about 40 kilometres from São Paulo's city centre and measures roughly 3.6 kilometres in diameter. Its distinctive circular shape was first documented scientifically in 1961, but until now scientists had not been able to prove how it formed.
The new study identified microscopic deformations found only in high-energy collisions inside zircon crystals collected from the crater's subsurface. Zircon is a mineral highly resistant to weathering and chemical alteration, which makes it useful for preserving evidence of extreme events over long periods.
Evidence recovered from deep underground
To establish the crater's origin, scientists analysed samples taken from deep boreholes drilled inside the structure. Beneath a sedimentary layer up to 275 metres thick, the team recovered fragmented rocks known as impact breccias and suevites, both of which form only under the extreme conditions produced by a collision.
The core of the research focused on the mineralogical analysis of zircon grains. Using high-resolution polarised light microscopy, the scientists mapped deformation microstructures in 40 selected crystals. The results showed planar deformation features and granular textures in the zircon, changes that only occur under the extreme pressure and temperature generated by shock waves from hypervelocity impacts.
Other geological theories ruled out
The investigation formally excluded volcanic activity or local tectonic movement as explanations for the Colônia formation. According to the study's authors, regional tectonic forces are incapable of producing the level of deformation observed in a mineral as mechanically stable as zircon.
The researchers also found no deep metamorphic associations or new indicator minerals in the host rocks, which rules out the possibility that the deformation resulted from later metamorphism rather than an impact.
Other shock indicators found at the same stratigraphic level reinforced the impact theory. These included planar deformation features in quartz, which require transient pressures far higher than any tectonic process can generate, mechanical deformations known as kink bands in mica, and spherules rich in iron, magnesium and silicon. The spherules are interpreted as condensed dust from the vaporisation plume produced by the impact.
A rare, well-preserved record
The Colônia Crater stands out as an unusual case among deeply eroded impact craters because of how well it has been preserved. Its sedimentary infill protected the deformed structures over time, turning the site into a conserved geological record within the São Paulo metropolitan region.
The researchers say the study demonstrates the reliability of zircon as a tool for identifying ancient, eroded craters and offers a standard method that could be used for future structural and geological reconstructions of impact sites elsewhere in the world.
